100 kW Electric Microgrid

Some of you have asked about our Microgrid module and how it could be used to power a microgrid. Here’s the details. I envision the smallest microgrid plant to produce 100kW of electrical power. Larger plants would use multiples of 100 kW, for example a one MW plant would be made of ten 100 kW plants. This is on the small size for current turbogenerators because efficiency drops quickly if they are even smaller. A 100 kW plant can power 20 US homes at 5 kW per home.

The big reasons for picking a small plant is the land area needed and the engine size needed for efficient mass production. Since we are partnering with the Xiang Yang Institute in China, we want the size to be consistent with the area needed for the solar array. A 100 kW electric plant would require 300 of our Microgrid collectors, each requiring 4 square meters of land area. That’s 1200 square meters for the solar array and another 400 square meters around the array for the turbogenerator facility, a periphery walkway and a security fence. At 1600 square meters, the land required is 40 m on a side. That’s about a half acre or 4 tennis courts. Smaller plots are easier to find than larger ones.

A second reason for a small plant is the turbogenerator size. A 100 kW unit – about 130 horsepower -- is the size of a truck engine. That’s something that can be made easily on an assembly line. More important, its turbine can be made on machine tools of reasonable size. China has trained tens of thousands of NC (Numerical Control) machinists in the 500 vocational training centers set up in small cities throughout China. It's these NC machinists that could mass-produce the 100 kW turbogenerator.

Back to the microgrid solar array, each collector covers a length of 2 m (79”) x 2 m (79”) where the width includes 1.2 m (47”) of module. Each row of modules requires a 0.8 m (30”) service access walkway between rows. That’s where the 4 square meters per module comes from. Each Microgrid module is raised 2 m (79”) above grade level. Beneath the module is thermal storage. While we are also looking into various types of phase change storage, the simplest heat storage is concrete where heat is stored as “sensible” heat. Sensible heat is the heat required to raise the temperature of the concrete; no heat of fusion or molten salt is involved. Here’s a schematic of how concrete could be used as a storage material for the Microgrid module.

Below each module is a horizontal cylinder of concrete 600 mm (24”) in diameter and 1.8 m (72”) long. Heat transfer pipes pass through each cylinder to both add and withdraw heat from it. Each cylinder is supported on concrete blocks to reduce its conduction heat loss to the ground. The cylinder is surrounded by fiberglass batting (glass wool) insulation to prevent convective and radiation heat loss from the cylinder. A cover protects the storage and insulation from the weather. Essentially each cylinder is thermally isolated from its surrounding.

Microgrid modules are “daisy-chained” together in long rows in the North South direction. The output of one module’s absorber flows directly into the next module’s absorber. Flexing unions keep thermal expansion stresses low. In a similar way, the storage cylinders are daisy-chained together. The heat transfer pipes of one cylinder flow into the heat transfer pipes of the adjacent cylinder. Again, flexing unions reduce thermal expansion stresses.

At each end of the module string, the absorber pipe is connected to its associated storage cylinder. A heat transfer loop is formed where mineral oil pumped through the Microgrid absorbers collect solar energy. Each module in the string adds solar energy to the oil, increasing its temperature. At the end of the string, the oil is hottest. There it flows down into the storage cylinders where it transfers heat to the concrete. As hot oil flows through each successive cylinder in the string, it loses its heat to the concrete. Arriving at the beginning of the string, the oil is pumped once again through the modules’ absorbers.

The entire loop is called the “solar loop” because it stores solar heat. Oil flowing through successive absorbers gets hotter and hotter until the end of the string. There it reverses direction and flows through the concrete cylinders. Heat is lost to each cylinder in succession until the oil is at its coolest at the beginning of the loop.

Heat is removed from the concrete by oil flowing through a second set of heat transfer tubes called the “user loop”. Mineral oil pumped through this second loop starts at the same module as the solar loop. As its oil passes through each storage cylinder in turn, it gets hotter and hotter. It is hottest leaving the last cylinder where it flows to the turbogenerator.

There the oil flows through a heat exchanger to heat the turbogenerator’s working fluid, converting its heat to electricity in a thermodynamic cycle. After leaving the turbogenerator, the oil is still hot, on the order of 100C. This “low grade” heat can be used locally to double the plant’s return on investment. For each kW-hour of electrical energy produced by the plant, 3 to 4 kW-hours of low grade heat is available from the turbogenerator. The heat can be used commercially for heating hotels and restaurants, district heating, laundries and air-cooling. Industrially it can be used for thermal desalination, absorptive refrigeration, food processing, fabric processing and other process heating applications.

The two loops – solar loop and user loop – act like a counter-flow heat exchanger. The module absorbers have their highest temperature at the last module in the string. The storage cylinders have the same arrangement: the temperature is hottest at the last module. The arrangement assures that the turbogenerator receives the hottest oil available.

While our Chinese partners are considering various turbogenerators to generate the Microgrid’s electricity, the simplest one available is an Organic Rankine Cycle engine or ORC engine. An Organic Rankine Cycle differs from its more common cousin, the Rankine Cycle, by the working fluid used. Steam (gas phase water) is the working fluid of the Rankine Cycle. It is the standard for generating electricity in most of the world. Organic Rankine Cycles use other molecules than water as its working fluid. ORC engines are available from many sources including German manufacturer Siemens and Japanese manufacturer Mitsubishi through their Italian subsidiary Turboden.

For you thermodynamics folks, we expect the temperature range of the storage will vary between 200C and 300C. The hotter the better, of course, if we want the highest Carnot efficiency. We think the Microgrid modules can deliver 300C (572F) heat at useful flow rates using vacuum jacketed tubes (see http://www.focused-sun.com/fs/technology/hybrid_absorber). As the storage temperature is depleted, the delivery temperature drops to perhaps 200C (392F) during normal operation. Most Microgrid arrays will have a backup generator to handle the possibility of a week of cloudy weather. Backups can be diesel generators, biomass generators or even boilers that add heat to storage when little solar energy is available.